linux-kernel - Re: [PATCH 1/1] uprobes: Fix the memory out of bound overwrite in copy

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Date:	Fri, 8 Nov 2013 17:24:15 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	David Long <dave.long@...aro.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] uprobes: Fix the memory out of bound overwrite in
	copy_insn()

On 11/07, Oleg Nesterov wrote:
>
> Note: we do not care if offset + size > i_size, the users of
> arch_uprobe->insn can't know how many bytes were actually copied
> anyway. But perhaps this needs more changes.

I guess this not is not very clear...

What I tried to say. Suppose that offset == 0 and i_size = 1.
In this case we still copy MAX_UINSN_BYTES into arch.insn
because the task gets the same page after page fault anyway,
and we can't inform arch_uprobe_analyze_insn() that we have
only read 1 byte. So this doesn't matter.

I changed this as follows:

	Note: we do not care if we read extra bytes after inode->i_size
	if we got the valid page. This is fine because the task gets the
	same page after page-fault, and arch_uprobe_analyze_insn() can't
	know how how many bytes were actually read anyway.

I also moved the comment up, before the main loop.

Srikar, I am going to add this to my tree unless you object now.
I verified that this indeed fixes the crash and the code looks
"obviously correct, see below.

Oleg.

static int __copy_insn(struct address_space *mapping, struct file *filp,
			void *insn, int nbytes, loff_t offset)
{
	struct page *page;

	if (!mapping->a_ops->readpage)
		return -EIO;
	/*
	 * Ensure that the page that has the original instruction is
	 * populated and in page-cache.
	 */
	page = read_mapping_page(mapping, offset >> PAGE_CACHE_SHIFT, filp);
	if (IS_ERR(page))
		return PTR_ERR(page);

	copy_from_page(page, offset, insn, nbytes);
	page_cache_release(page);

	return 0;
}

static int copy_insn(struct uprobe *uprobe, struct file *filp)
{
	struct address_space *mapping = uprobe->inode->i_mapping;
	loff_t offs = uprobe->offset;
	void *insn = uprobe->arch.insn;
	int size = MAX_UINSN_BYTES;
	int len, err = -EIO;

	/* Copy only available bytes, -EIO if nothing was read */
	do {
		if (offs >= i_size_read(uprobe->inode))
			break;

		len = min_t(int, size, PAGE_SIZE - (offs & ~PAGE_MASK));
		err = __copy_insn(mapping, filp, insn, len, offs);
		if (err)
			break;

		insn += len;
		offs += len;
		size -= len;
	} while (size);

	return err;
}

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